1. Field of the Invention
The present invention relates to methods of rate detection at a receiving end of a digital communications system, such as a code division multiple access (CDMA) system, in which system the information data rate is variably selected at the transmitting end from an applicable rate set including a full rate and lower rates, each lower rate being the full rate divided by a different integer, and data is repeated for the lower rates to maintain a constant apparent data transmission rate. In its particular aspects, the present invention relates to a rate detection method in which a rate determination or classification decision process uses measurements of repetition characteristics of data which has not been de-repeated.
2. Description of the Related Art
Such a rate detection method is generally known from Edith Cohen and Hui-Ling Lou, xe2x80x9cMulti-Rate Detection for the IS-95 CDMA Forward Traffic Channels, IEEE Global Telecommunications Conference 1995.
xe2x80x9cIn 1992, a direct sequence code division multiple access (DS-CDMA) system was adopted as Interim Standard 95 (IS-95) by the Telecommunications Industry Association (TIA) for deployment in the cellular band at 800 MHz. After successful field tests and trial systems, the IS-95 system is now operating with tens of millions of subscribers.xe2x80x9d
CDMA is based on spread spectrum technology originally developed by the Allies during World War II to resist enemy radio jamming. Spread spectrum signals are characterized by a bandwidth W occupied by signals in a channel much greater than the information rate R of the signals in bit/s. Thus, a spread spectrum signal inherently contains a kind of redundancy which can be exploited for overcoming several kinds of interference (including signals from other users in the same band and self-interference in the sense of delayed multipath components) introduced by the channel. Another key property of spread spectrum signals is pseudo-randomness. Therefore, the signal appears to be similar to random noise, making it difficult to demodulate by receivers other than the intended ones. In CDMA systems, users share a common channel bandwidth and users are distinguished by different code sequences. In the case of IS-95 each communication with a user is modulated or scrambled by long and short Pseudo Noise (PN) sequences and also modulated by a specific one of a set of orthogonal sequences, known as Walsh codes, which is assigned to the user. The latter modulation is known as applying a Walsh cover. Thus, a particular receiver can recover a certain transmitted signal by applying the PN sequences, and also the Walsh sequence used by the corresponding transmitter for the particular receiver.
In the IS-95 DS-CDMA system variable information data rates are used according to the voice activity detected by the voice encoder. This enables a reduction in transmitted power at the lower rates leading to a reduced average transmitted power per user and consequent increase in capacity of the system. Two sets of information data rates (Rate Sets 1 and 2) can be encoded, depending on the implemented voice encoder each set comprising full rate, and lower rates of half rate, quarter rate, and eighth rate. For the lower rates, symbols are repeated to achieve the same apparent symbol transmission rate as when full rate is used. In Rate Set 2, there are 50% more symbols in a frame than in Rate Set 1, but prior to transmission one third of the Rate Set 2 symbols are punctured so that in both rate sets the same number of symbols in a frame are transmitted. The information data rate can change from frame to frame, but information indicating the currently used data rate is not transmitted along with the speech data. Therefore, the receiver has to detect the data rate by hypothesis testing. The algorithm implemented by rate classification or decision logic which determines which of the possible information data rates is utilized for the current frame is called a Rate Detection Algorithm (RDA).
In accordance with the known rate detection method utilizing repetition characteristics of data, prior to any de-repetition, measures are formed for Rate Set 1 which determine how well symbols match within successive groups of 2, 4, and 8 symbols. Such method, while requiring only a relatively small amount of computational resources, is not of sufficient reliability for an IS-95 DS-CDMA system that a rate decision could be based solely thereon. Further, such known method yields even poorer results when applied to Rate Set 2, because it does not take into account the effects of puncturing.
Other information which could be used for rate detection includes CRC checking results (which in accordance with IS-95 are available for all data rates except quarter and eighth rates in Rate Set 1), Viterbi decoder survivor metrics, and correlations between re-encoded data and data entering the decoder for each possible data rate. The latter two methods, which utilize data available from or after the Viterbi decoding at each of the possible data rates, and for each data frame, are inherently more reliable, but are computationally intensive. The method employing such correlations makes particularly intensive use of computational resources since, each frame, for each of the possible data rates the data must not only be Viterbi decoded (after de-puncturing and de-repeating as required), but also convolutional re-encoded in order to form correlations between the re-encoded data and the data entering the Viterbi decoding for each possible data rate.
Intensive use of computational resources is undesirable, particularly in wireless handsets, because battery life is generally reduced as the number of instructions per section required to be executed by a digital signal processor (DSP) within the handset increases. A significant power savings is gained when the DSP has a relatively high proportion of slack time, during which the DSP can go into an idle mode.
It is an object of the present invention to provide an improved method of rate detection at a receiving end of a digital communications system which is highly reliable but on average utilizes a relatively small amount of computational resources. It is a further object that the rate detection method take into account any pattern of puncturing used for an applicable rate set.
These and other objects of the present invention are satisfied by such a method of rate detection wherein the data rate of received convolutional encoded data is first determined by a coarse decision method which is computationally simple because it is based on measures of the data computed before any Viterbi decoding (xe2x80x9cpre-decodingxe2x80x9d measures). Then, using data available from or after the Viterbi decoding at the first determined data rate to obtain or form a xe2x80x9cpost-decodingxe2x80x9d measure, an evaluation is made whether the first determined data rate should be selected as the actual data rate, and preferably, also whether there is high or low confidence in this selection. Only when, the evaluation does not result in the selection of the first determined data rate as the actual data rate, is a more accurate but computationally intensive fine decision method resorted to, using post decoding measures obtained or calculated at one or more other data rates.
In accordance with the invention, pre-decoding measures of repetition patterns in the data are calculated for each of the possible lower data rates, and the coarse rate decision is made using these measures and a first set of thresholds. In the calculation of the pre-decoding measures for Rate Set 2, it is taken into account that the data has been punctured according to a predetermined pattern.
In order to evaluate the result of the coarse decision method, the convolutional encoded data is de-punctured and de-repeated where required, Viterbi decoded, and convolutional re-encoded, all with respect to the first determined data rate. Then there is first formed a correlation between the received convolutional encoded data after any de-puncturing and de-repetition at the first determined data rate and the convolutional re-encoded data, and this first formed correlation is compared with a predetermined threshold associated with the first determined data rate, this threshold being contained in a second set of thresholds. The actual data rate is selected as the first determined rate when the first formed correlation is greater than (or is equal to) this threshold.
If on the other hand the first formed correlation is less than the threshold to which it is compared, the fine decision method is applied wherein the received convolutional encoded data is second Viterbi decoded in accordance with at least one other data rate of the rate set, after de-puncturing in the case of Rate Set 2, and de-repetition if the at least one other data rate is one of the lower rates, and the data rate is second determined utilizing data available from or after the second Viterbi decoding.
Further, in accordance with the present invention, the fine decision method preferably uses correlations formed between the received convolutional encoded data, after any de-puncturing, and any de-repetition, and convolutional re-encoded data, at the full rate and each lower data rate in the applicable rate set, wherein data rates are considered beginning with the full rate, and the determined data rate is set equal to the considered rate when the correlation formed at the considered data rate satisfies a set of one or more conditions
One of the conditions is the correlation formed at the considered data rate plus a second predetermined threshold associated with the considered data rate being greater than a largest of the correlations formed at the other data rates. A second condition is, when Cyclic Redundancy Code (CRC) checking is available for the considered data rate, that a CRC check with respect to the decoded received convolution encoded data for the considered data rate does not fail. If CRC checking is not available for the considered data rate, the second condition is the correlation formed for the considered data rate being greater than a third predetermined threshold associated with the considered data rate.
Since the coarse decision method provides the correct rate decision most of the time, and the fine decision method need only be applied a small fraction of the time, the rate determination method of the present invention uses only slightly more computational resources than the coarse decision method but has the reliability of the fine decision method.
Other objects, features and advantages of the present invention will become apparent upon perusal of the following detailed description when taken in conjunction with the appended drawing, wherein: